If a merge block is reachable, then it must be *strictly* dominated
by its header. Until now we've allowed the header and the merge
block to be the same.
Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/551
Also: Use dominates and postdominates methods on BasicBlock to
improve readability.
According to the SPIRV Spec (2.16.1):
* There is at least one OpEntryPoint instruction, unless the Linkage
capability is being used.
* No function can be targeted by both an OpEntryPoint instruction and an
OpFunctionCall instruction.
Also updated unit tests to includ OpEntryPoint.
entry_block_to_construct_ maps an entry block to its construct. The key
in this map (the entry block) is not unique, and therefore the entry for
the continue construct gets overwritten when the selection construct is
discovered.
Since a given block may be the entry block of different types of
constructs, the (basic_block, construct_type) pair should be able to
uniquely identify the construct.
Adds test:
- In this test, a basic block is the entry block of a continue construct
as well as the entry block of a selection construct.
It can be shown that this unit test would crash without the fix in this
PR and passes with the fix in this PR.
The number of (literal, label) pairs passed to OpSwitch may not exceed
16,383. Added code to validate this and added unit tests for it.
Also fixed a typo in another validor error message.
